The advantages of this method of origination are quite evident. Since 180 blocks were made to fill the circle, the edges of each had 2° taper. This taper enabled the blocks to be fitted perfectly to the template, because any error in fit would be remedied by letting the block farther down into the template. Hence, it was possible to correct any error that was discovered without throwing the block away. Further, as the blocks themselves are removed to form a recess for locking the originating wheel in position while cutting the worm-wheel, the truth of the work is not subject to the errors that creep in when holes or notches require to be pierced in the originating wheel. Such errors arise from the heating due to the drilling or cutting, from the wear of the tools or from their guides, from soft or hard spots in the metal and other similar causes. To avoid any error from the heating due to the cut on the worm-wheel, in producing master wheels, Messrs. Hoe and Co. allowed the wheel to cool after each cut. The teeth were cut in the following order: The first three were cut at equidistant points in the circumference of the wheel. The next three also were at equidistant points, and midway between those first cut. This plan was continued until all the teeth were cut, thus making the expansion of the wheel from the heat as nearly equal as possible in all directions.
There is one feature in this plan that is of value. It is that a certain number of blocks, for example six, may be taken out at two or three different parts of the originating wheel and interchanged, thus affording a means of testing that does not exist in any other method of dividing.
The tools applied by the workmen to measure or to test work may be divided into classes.
1st. Those used to determine the actual size or dimension of the work, which may be properly termed measuring tools.
2nd. Those used as standards of a certain size, which may be termed gauges.
3rd. Those used to compare one dimension with another, as in the common calipers.
4th. Those used to transfer measurements or distances defined by lines.
5th. Those used to test the accuracy of plane or flat surfaces, or to test the alignment of one surface to another.
Referring to the first, their distinctive feature is that they give the actual dimensions of the piece, whether it be of the required dimension or not.
The second determine whether the piece tested is of correct size or not, but do not show what the amount of error is, if there be any.